The present invention relates generally to devices for determining the direction of the earth's magnetic field, e.g. to magnetic compass devices. Such devices are used for finding the direction of the earth's magnetism at a particular point on the earth's surface or in the air. The output of such sensors may not only be used for finding the direction of the magnetic field but also, by reference to a known distribution of the earth's magnetic field in all three dimensions, for identifying the location of the sensor on the surface of the earth. Devices as the magnetic field sensor device according to the present invention may serve for automobiles, airplanes or as mobile devices independent of any transportation medium.
FIG. 1 illustrates a known earth's magnetism flux sensor with an O-ring-shaped magnetic core C made of permalloy. On the magnetic core C an exciting coil CD and detection coils CX and CY perpendicular to each other are wound. An oscillator OSC provides a signal having a frequency f to the exciting coil CD. In the detection coils CX and CY a signal is thereby induced wherein the signal is also depending on the horizontal component of the earth's magnetism in parallel to the longitudinal axis of the respective detection coil CX or CY. The outputs of the detection coils are applied to respective synchronous detectors SYNC through respective bandpass filters BPF and respective amplifiers AMP. The synchronous detectors SYNC also receive the reference signal having the frequency 2f provided by said oscillator OSC through the frequency doubler DOUBLER and phase controller PHASE CONT. Respective outputs of the circuit EX and EY relate to the direction of the coils CX and CY with respect to the earth's magnetic field.
If the coils CX and CY are positioned horizontally, the horizontal components of the earth's magnetic field are detected.
When the sensor device is rotated about a vertical axis by 360 degrees, the intensity of the earth's magnetic horizontal field components detected by the respective coils CX and CY follows a sinusoidal curve shown in FIG. 2.
Another embodiment of a magnetic field sensor device shown in U.S. Pat. No. 4,739,263 avoids some disadvantages of the device described above as for example the large size and drifts in measurement over a period of time. This is achieved by a device with separate magnetic cores for the distinct directions, where for each direction two coils are located on opposite ends of a single magnetic core. The coils are excited by a signal having a frequency f and a differential level of output of the two coils is measured by an electric circuit, the output being balanced to zero as long as no external magnetic field is present. As soon as the earth's magnetic field component parallel to one of the magnetic coils is present, the respective differential level is out of balance and this imbalance is measured.
This magnetic field sensor device is much more sensitive to magnetic fields than the one described above and less sensitive to drifts.
Additionally, a third sensor element may be provided which is the same as those used for the horizontal components and which is appropriately positioned to detect the vertical component of the earth's magnetic field. According to the prior art, this third sensor element is used to align the other two sensor elements perpendicular to the vertical axis.
From U.S. Pat. No. 2,852,859 a device for determining the direction of the earth's magnetic field in the form of a flux valve is known. This magnetic field detector consists of a sensitive core element in form of a spider of high permeability metal having a generally Y-shaped configuration, the legs or arms of which are preferably arranged 120 degrees apart. Suitable horns of the permeable material are provided for collecting the magnetic lines of the flux and concentrating them in the legs of the spider.
The spider is provided with a centre winding which is excited electrically with an alternating current supply voltage of frequency f which serves cyclically to vary the reluctance of the core material, that is to periodically saturate and unsaturate the core material and thereby effectively open and close the valve to any magnetic field in the vicinity of the valve. Mounted on each leg above the spider is a pick-up or output winding in which is generated a voltage each time the valve is opened and closed by the excitation voltage. Thus the output of each winding will be an alternating voltage proportional in amplitude to the magnitude of the component of the external magnetic field which is arranged substantially collinear to the axis of the respective pick-up coil. The leads of the pick-up coils are connected to an electrical circuit for analysing the measurements.
European Patent Application EP 1037304 A2 discloses a sensor element comprising a magnetic core and a winding, said sensor element preferably serving as an antenna for a car locking and unlocking system which allows remote locking or unlocking of car doors. Said sensor element has a structure that allows for miniaturizing and low cost mass production.
It is an object of the present invention to overcome the disadvantages and limitations of prior art magnetic field sensor devices by providing a new and improved device.
It is also an object of the present invention to provide a magnetic field sensor device which is small in size, light in weight, easy to manufacture, inexpensive and has a high operational reliability.